Identifying plastics

Why identify the material?

It is good practice to identify the material of which an object is made because it enables you to understand more about the object. But it is vital for objects made of plastics as it will help you know how best to look after them.

All plastics degrade over time but some are much less stable than others. It makes sense to concentrate limited resources on providing objects made of these plastics with optimum environments or you might even decide not to collect such objects at all. To find out which materials these are go to Problem plastics.

The best way to learn to identify different plastics is to study a group of objects that already have the plastic from which they are made identified. That way you can get to know what they look, feel and smell like. Clues to help you know what to look for can be found at Identification: clues to get you started.

Identification can also involve sophisticated analytical equipment of which the Fourier Transform Infrared Spectrometer (FTIR) is the most widely used. Such machines can be brought to your museum at a cost. It does however require experts to carry out the analysis and opinions differ on its value.

For information on this and other forms of instrumental analysis please refer to Anita Quye and Colin Williamson ed., Plastics collecting and conserving, part two: analytical methods, pp.70 -73.

Clues to get you started

There will always be something you can glean from an object itself to help you decide what material it is made of or how it was manufactured. If you have any thoughts to contribute to the questions below click on them and find out how what you know may help. The notes attached to each question aim to help you make the most of what you know about the object to narrow down the options. Once you have done that you can go to the particular materials in the A –Z of plastic materials or the particular manufacturing processes in the A - Z of manufacturing processes to look in more detail at what you have decided are the probabilities.

When was it made?

If you have an idea when the object was made, use the information under the relevant date span to narrow down the probabilities. Bear in mind though what you are getting are probabilities not certainties. Many plastics have had long periods of gestation and, as more and more plastics are invented, some become outmoded but nonetheless stay in production. And, although some materials are used most often with a particular manufacturing process, they may also be used from time to time with another. If you have a hunch that an object is made of a particular material outside the dates given or manufactured in a different process go to the material or process in the A–Z guides to check out what is possible in greater detail.

What does it look like?

Transparent

Relatively few plastics are transparent like glass. All transparent plastics can be made translucent or opaque by the addition of pigments or fillers. Some plastics are only transparent in sheet form. If it is moulded and transparent it is probably made of one of the following:

Physical distortion, warping

What marks are on it?

A small bird’s wing was used to indicate the use of the material bois durci.

An infinity sign is the logo of Bakelite and thus frequently indicates the material phenol formaldehyde but the company made many other plastic materials. It only appears on Bakelite promotional mouldings. Bakelite did not make mouldings for the general market.

Recycling triangles were introduced in 1988 so any object with these on must date from that year or later.

Smooth circular marks are a sign of the use of ejector pins to push the moulding from the mould and thus of injection moulding.

An imperfection on an otherwise smooth surface may be a residue left at the spot the material has been forced into the mould and thus indicate the use of injection moulding. Such marks can be extremely hard to detect and they may not be where you might expect to find them, for example centrally placed on the base or on the edge. They can be polished off so their absence does not tell you anything.

The following are trade names that frequently appear on mouldings. They are associated with the materials indicated: